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Full text of "Handbook Of Chemical Engineering - I"

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288                                 CHEMICAL ENGINEERING
water glass by acids in a way that creates an enormous amount of ultramicrospic surface excellently affording in small compass an enormous surface power. As one authority has expressed it "If you consider the measure of the gel drop in centimeters, you must measure the area of the cracks in acres." This may give one clue to the adsorptive power of diatomaceous earth. Another clue to power of this kind in analgous substances may possibly be obtained from the following abstract of an article by Milton Whitney, Bureau of Soils, U. S. Department of Agriculture, appearing in Science, Oct. 14,1921, p. 348, and taking up the subject of "Ultra Clay" isolated from ordinary clay. This ultra clay which appears to be a silicate of aluminum and iron will when dry absorb as much as 200 times its volume of ammonia gas, from 20 to 40 times of its weight of water vapor, and from 10 to 30 per cent of its weight of certain dyes. By heating the ultra clay to 900 to 1,000°C. this absorption power is practically completely killed. This ultra clay is as strong in its power of cementing sand grains as is Portland cement but when a dry briquette cemented by it is put into water it goes to pieces. The hardening of roads after a rain is attributed to it and their mushiness and slipperi-ness while wet.
Separation by Direct Electric Current.—This mode of separation forms the basis of numerous patents by Count Botho Schwerin (cf. "Control of Ore Slime," Oliver C. Ralston, Eng. and Min. Jour., April 29, 1916). Many clay suspensions can be separated by utilizing the fact that particles of them suspended in water will migrate towards one or the other of the two electrodes introduced into the suspension. Direct current must be used and as clay particles will be negatively charged, they migrate towards the anode. In the purification of clays (cf. Ormondy, Trans. Eng. Ceramic Soc., XII, pp. 36-64 (1912-13) and Bleininger, Trans. Am. Ceramic Soc., XV, p. 335 (1913)) the impure clay is made into a slip of the consistence of thick cream and is preferably deflocculated by use of sodium hydrate. Coarse particles are settled out as well as some of the iron minerals. Following the settling operation the clay suspension is run into a metal trough into which dips a revolving cylinder made of lead' or type metal. This is the anode. The current voltage ranges from 110 to 220 volts. The clay forms a blanket on the anode and after it revolves out of the liquid the clay is removed by a scraper. The clay contains only 17 per cent moisture which is less than would result with filter pressing under a pressure of 2 tons per square inch. The slip must be stirred during the separation operation in order to bring as many particles in contact with the anode as possible. According to the English authority cited the cost of current for depositing a ton of clay is in England from 10 to 64 cts. and a plant capable of producing 40 tons a week will cost $25,000. Successful use of this means of separation requires that the material to be separated be in an extraordinarily fine state of subdivision. Ralston reports that finely divided ores do not separate well by this method and attributes the failure to the fact that the ore particles are in too coarse a state of subdivision to be affected by the feeble motive force of the current. The successful separation of ground flint and kaolin and the failure with feldspar and ferric oxide described in the experiments below raises the query as to whether ores were tried which contained large proportions of the second pair of substances or other ones which do not separate well.
The movement of small particles under the influence of a direct current either to the cathode or the anode is called cataphoresis. Suspended particles of cotton, silk, starch, graphite, sulphur, etc., travel to the positive electrode. In pure water